Dental prostheses are prosthetic appliances that replace missing teeth to aid mastication and improve facial esthetics, phonetics, and self esteem. Removable dental prostheses include dentures, partial dentures, and detachable fixed prostheses. The traditional methods of fabricating removable prostheses rely on alginate or silicone impressions of bone, soft tissue structures, and the dentition of a patient. These methods usually require multiple patient visits to obtain a clinically acceptable impression. The impressions are poured with dental stone to obtain model casts or replicas of the upper and lower jaws. Model casts or casts are plaster replicas of dental structures obtained from impressions of the dental structures.
A proper clinical step in prosthodontics requires a face bow transfer that records the position of the upper teeth in relation to the maxillary jaw. In addition to the face bow transfer records, the bite registration of the upper jaw and the lower jaw while the mandibular condyle is placed in functional positions such as the centric relation is also taken. Due to the involvement of the masticular muscle groups, there have been controversies over the definition and method of obtaining accurate occlusion data.
The face bow transfer and bite registration are then used to mount the upper and lower jaw casts onto a semi-adjustable mechanical articulator that simulates the mandibular jaw motion. The semi-adjustable dental articulator has a fixed or semi-adjustable condyle portion, a fixed lower jaw portion, with a simplified limited operation of jaw motion. These mounted casts of the upper jaw and the lower jaw are sent to dental labs, where prosthetic teeth are set in wax form for a patient's try-in. At the try-in appointment, the wax prosthesis is examined for esthetics, occlusion, and speech. Esthetics usually requires a proper smile profile, for example, with the teeth showing when smiling, and around 1 mm-2 mm of the teeth showing at the resting position of the upper lip. Occlusion requires proper contacts between the upper teeth and low teeth. Speech requires a proper seal between the upper teeth and the lower lips.
After a number of try-ins, the final wax form with properly set teeth is returned to the dental labs for processing. While the teeth are held in place by investing materials, the wax portion is replaced by pink tissue-colored acrylic materials and polymerized under high pressure and temperature. The polymerized products are then trimmed and polished to develop the finished prostheses. The entire process typically takes one to two months. Moreover, these prostheses require extensive adjustments and multiple additional visits, and also do not guarantee patient satisfaction.
While every step in the dental clinic and the dental lab requires tedious and precision-driven labor, human errors are inevitable. Furthermore, there are many inherent sources of errors that are difficult to control, for example, soft tissue deformation during impression, limitations of the semi-adjustable articulator, controversies over proper form of occlusion, and shrinkage of acrylic material during polymerization.
A major problem involving removable prosthesis is the retention of the prosthesis. Although the acrylic material adapts to the shape of the plaster cast model initially, the acrylic base shrinks and deforms during the polymerization process, thereby losing its precise seal to the plaster model and the soft tissue. If the base of the prosthesis does not seal tightly to the soft oral tissues, food particles and air can seep into the space under the prosthesis and make it less retentive. On the other hand, over retention due to lack of precision in metal framed partial dentures is also a problem. Over retention not only causes pain and discomfort, but also damages the anchoring teeth in the long term. Due to the lack of technology to precisely shape the base or anchor area of the prosthesis, current methods mostly rely on multiple clinical adjustments by the dentist using pressure-indicating paste, which still does not warrant a tight seal or fit to the tissue surfaces. Dentists often spend significant time adjusting the base of the prosthesis and patients are generally unhappy with the outcome. Another problem is improper occlusion due to inaccurate face bow transfer or the absence of a face bow transfer. This results in unbalanced occlusion interference, improper teeth alignment, or insufficient occlusion space, which further reduce the retention. As a result of these unresolved problems, the service of removable prostheses is currently under-valued and under-served in the dental industry.
Hence, there is a long felt but unresolved need for a computer-aided method and system for designing and fabricating a highly retentive, functional, and esthetic removable dental prosthesis for a patient.